A pressure-sensitive adhesive can be defined as a material which, in dry form, is aggressively and permanently tacky at room temperature so that it may firmly bond a variety of dissimilar surfaces upon contact without need of more than finger or hand pressure (low pressure). Molecular weights of polymers used as pressure-sensitive adhesives cover a broad range depending on their type, composition, structure, and method of polymerization. They are available in a variety of base chemistries and advantageously are formulated to have a particular balance of tack, adhesive, cohesive, and elastic properties, together with good thermal and chemical stability. They are available both as emulsion (latex) and solution polymers. Properties such as bond strength, shear strength, and tack may be adjusted by changing the molecular weight and chemistry of the polymers or by adding fillers or plasticizers.
The tack of the adhesive refers to its ability to form an instantaneous bond by flowing and wetting-out of the substrate with virtually no applied pressure. Tack can be measured by a variety of methods which are known in the field including loop tack, rolling ball tack, or the like. A number of test methods known in the field are identified as the Pressure Sensitive Tape Council (PSTC) Test Methods, which include a PSTC-5 quick stick tack test. The adhesive properties refer to the ultimate bond realized over a time frame under a specified lamination pressure. A PSTC-1 test comprises a 180 degree peel adhesion test using stainless steel panels and a four and one-half pound rubber roller for contact pressure. Adhesion tests are frequently carried out after 0, 15 minute, 24 hour, 72 hour, and 168 hour dwell times at specified conditions of temperature and humidity. An increase in adhesion with time is indicative of the relative "wet out" of the adhesive. The cohesion reflects the internal strength of the pressure-sensitive adhesive and is measured by shear strength tests, such as PSTC-7 (a dead load shear test) and other tests known in the field including lap shears, shear adhesion failure temperature (SAFT), and Williams plasticity (compression resistance).
Acrylic pressure-sensitive adhesives are soft, permanently tacky polymers preferably fabricated to have glass transition temperatures (T.sub.g) of about -15.degree. C. to -55.degree. C., as discussed in U.S. Pat. No. 3,579,490 issued May 18, 1971, to Kordinzinski et als., entitled "Method of Producing Resins for Use in Adhesives," which is hereby incorporated by reference. The glass transition temperature (T.sub.g) is the temperature at which the polymer changes from a hard, glassy material to a soft, rubbery material. Acrylic pressure-sensitive adhesives have specific attributes that increase their utility in various applications. Their beneficial attributes include resistance to oxidation and ultra-violet radiation, high optical clarity with little or no color, high bond strength to a variety of substrates, and versatility of formulation for cohesive strength, heat resistance, and solvent or chemical resistance. Acrylic pressure-sensitive adhesives can be prepared with solution or latex polymerization techniques having approximate molecular weights of less than 10.sup.5 or greater than 10.sup.6 with solution polymers comprising lower-molecular weight polymers.
As a result of their beneficial attributes, acrylic pressure-sensitive adhesives find utility in a wide variety of applications, including the graphic arts and for use in fabricating decals, labels, tapes, membrane switches, medical devices, and other protective and masking works. The flammability of adhesives is of concern in some applications, however, including electronic devices and appliances, for electrical tape, and in fabricating flexible optical circuits or multi-wire boards. A challenge with pressure-sensitive adhesives for use in such applications has been developing materials that have optimal or desired levels of adhesive properties and yet are non-flammable. Pressure-sensitive adhesives based on acrylates or polyacrylates, for example, have excellent pressure-sensitive adhesive properties, but they are also flammable.
One common approach in reducing the flammability of a pressure-sensitive adhesive is to blend combustion-inhibiting additives in the adhesive. Many flame-retardant additives contain bromine, such as brominated diphenyl or brominated diphenyl oxide compounds. Decabromodiphenyl oxide often is used, for example, which has good flame-retardant properties in light of its high bromine content. Another commonly-used additive is antimony trioxide, which may be used in combination with halides, such as titanium tetrachloride. Halogen radicals provided by these additives react to form hydrogen halides which interfere with the radical chain mechanism in the combustion process, thereby breaking the combustion cycle. Antimony acts as a synergist to increase the efficacy of the halides.
However, adding combustion-inhibiting materials may disrupt the sensitive balance of the properties of the adhesive, such as tack, cohesive strength, solvency, and stability. Typical flame retardant systems, such as those based upon decabromodiphenyl oxide and antimony oxide, tend to settle out of acrylic coatings and adhesives, and they opacify the polymer and detract from its adhesive properties.
Aside from the halogen-containing flame retardants, phosphates also have been used to develop flame retardants, particularly condensed-phase flame retardants in oxygen containing polymers. See M. Robert Christy, Standards, Bans and Flame Retardants, PLASTICS COMPOUNDING (September/October 1993), at 59. A phosphate-containing pressure-sensitive adhesive is disclosed in U.S. Pat. No. 3,515,578, issued Jun. 2, 1970 to Tomita, et al., entitled "Pressure Sensitive Adhesive Tape," and assigned to Minnesota Mining and Manufacturing Co (3M). The 3M patent describes pressure-sensitive adhesives involving polyacrylates modified by tris-(halogenated alkyl) phosphates and antimony trioxide, and describes as preferred tris-(halogenated alkyl) phosphates with dibromo-substituted alkyls having three carbon atoms and, in particular, tris(2,3-dibromopropyl)phosphates, although this phosphate has been determined to be carcinogenic and banned by the U.S. Environmental Protection Agency. The use of brominated-phosphates as a flame retardant is mentioned in Spotlight, Customer Demands: Synergistic Flame-Retardant Systems, PLASTICS COMPOUNDING (January/February 1994), which discloses a study relating to a compound comprising 16.7% brominated polycarbonate oligomer and 12% triphenyl phosphate or 6% brominated phosphate (60% bromine, 4% phosphorus). In this instance, the phosphates are used as fillers by blending in the polycarbonate compound.
Use of phosphates has been discouraged as adversely affecting the mechanical properties of the materials particularly when present as a filler. See Favstritsky et als., U.S. Pat. No. 5,100,986, entitled "Flame Retardant Brominated Styrene-Based Coatings," issued Mar. 31, 1992, which is hereby incorporated by reference. While exhibiting good flame retardancy and clarity, the phosphates tend to be insoluble in water, and when used in conjunction with a polymer, they tend to plasticize the polymer and migrate to the surface, depending on their compatability with the polymer and other additives, i.e., certain phosphates will greatly weaken the cohesive properties of the adhesives.
An advantageous approach for developing a flame-retardant pressure-sensitive adhesive is to react a flame retardant into the polymer backbone, as compared with using additive flame retardants blended in the polymer. There has been limited success with such integrated polymers, as discussed in Wang & Favstritsky, Flame-Retardant Brominated Styrene-Based Polymers, JOURNAL OF COATINGS TECHNOLOGY, Vol. 68, No. 853 (February 1996), pp. 41-47, at page 41. For example, copolymers of acrylonitrile, vinylidene chloride, and vinyl chloride are of this type and have both adhesive and fire-retardant properties. However, chlorine is less effective than bromine or phosphorous in producing flame-retardant properties and thus, such chlorinated compounds are not as effective as compositions incorporating bromine or phosphorous. Also, chlorinated polymers are less thermally and hydrolytically stable than brominated polymers.
The development of flame-retardant polymers involving dibromostyrene is described in Wang & Favstritsky, JOURNAL OF COATINGS TECHNOLOGY, cited above, and Wang & Favstritsky, Novel Flame Retardant Dibromostyrene-Based Lattices: Synthesis, Characterization, and Applications, presented at the Waterborne, High-Solids, and Powder Coatings Symposium (Feb. 22-24, 1995) (hereinafter "Symposium Paper"). Wang et al. discloses polymers of dibromostyrene and butadiene or dibromostyrene and a plurality of monomers selected from the group consisting of styrene, butadiene, methacrylic acid, ethyl acrylate, butyl acrylate, methyl methacrylate, 2-ethylhexyl acrylate, and itaconic acid, with the choice of monomer depending on the application. See Symposium Paper at 1. See also U.S. Pat. No. 5,066,752, to Favstritsky et al., issued Nov. 19, 1991, entitled "Flame Retardant Brominated Styrene-Based Polymers"; U.S. Pat. No. 5,100,986, cited above; U.S. Pat. No. 5,290,636 to Rose et als. issued Mar. 1, 1995, entitled "Flame Retardant Brominated Styrene-Based Coatings"; and U.S. Pat. No. 5,438,096 to Wang et als. issued Aug. 1, 1995, entitled "Flame Retardant Brominated Styrene-Based Latices," all of which were assigned to Great Lakes Chemical Corp. and are incorporated herein by reference.
One application for use of flame-retardant pressure-sensitive adhesives involves optical circuit devices. Optical circuits are tested for flame retardancy pursuant to standards known in the industry for measuring the flammability of plastics used in electronic devices and appliances, namely, the Underwriters' Laboratory (UL) 94 standards. The UL standards are well known and also are described in M. Robert Christy, Standards, Bans, and Flame Retardants, PLASTICS COOMPOUNDING (September/October 1993), at pp. 59-61. The UL 94 vertical (UL94V) standards have been applied to optical circuit devices, including the UL94V test and the 94VTM test, with the latter test (94VTM), applicable for thinner materials prone to distortion.
A difficulty with adhesives used in optical circuits has been developing materials that meet UL 94 ratings while maintaining desired levels of adhesiveness. For circuits to meet desired levels of flame retardancy, combustion-inhibiting additives at greater than twenty-five percent of the total solids would be required. This would decrease the tack of the adhesives to the point that they could no longer meet desired fiber placement tolerances. Adhesives used in optical circuit devices should have a peel strength of at least two pounds per inch; should have sufficient tack so a curved fiber with a radius of one inch will be held in place without allowing the fiber to relax and straighten itself out, and will be held in place to plus or minus 1 mil after being pressed into the adhesive at about a one-quarter pound force; should remain stable when exposed to standard environmental testing as is known in the industry; should not contain reactive constituents that might degrade the composite; and should not require the use of special procedures, such as gloves or ventilation, to handle the adhesive at temperatures up to 100 degrees Centigrade.
Accordingly, there remains a need for improved non-flammable pressure-sensitive adhesive in which flame retardants are reacted into the polymer backbone having high flame retardant properties with good adhesion, cohesion, and tack. There particularly remains a need for such an adhesive that may be used in fabricating flexible optical circuit devices. The adhesives of this invention satisfy this need, although they find utility in other applications as well, such as electrical tape or in electronic devices and appliances. Further advantages may appear more fully upon consideration of the description given below.